The staphylococcal enterotoxins are the most potent T-cell mitogens known, with staphylococcal enterotoxin A (SEA) capable of stimulating human and mouse T-cell proliferation and induction of the lymphokines interleukin 2 (IL 2) and gamma interferon (IFN8) at concentrations as low as 10-13-10-16 M. IL 2 and IFN8 are biological response modifiers (BRMs) that are being used in experimental cancer therapy. Structure/function studies of the potent enterotoxin effects on lymphocyte function should provide insight into the mechanism of action of these toxins and should also provide an approach for modulation of host defense against cancer. It is proposed that a novel synthetic peptide approach be used to determine the nature of the cellular binding/functional site(s) on staphylococcal enterotoxins. Both SEA and SEB will be studied in receptor competitive binding experiments with synthetic peptides corresponding to enterotoxin sequences that are predicted to appear on the exterior of the molecule. Identification of surface sequences is based primarily on predicted composite surface profiles that are derived from HPLC-derived hydrophilicity, accessibility parameters, and surface B-value parameters of the amino acids of the enterotoxins. In addition to competition studies, labeled peptides will also be tested for binding to receptor. Both monoclonal and polyclonal antibodies to the enterotoxins and the peptides will be tested for blockage of enterotoxin function and binding to receptor. Synthetic peptides will be used to map the epitope specificity of monoclonal antibodies to the enterotoxins that block function and/or receptor binding. Steric relationships of epitopes, and thus various regions of enterotoxin, can be assessed by competitive binding between monoclonal antibodies (and their Fab fragments) of defined epitope specificities with the enterotoxin molecule. Structure/function data on the enterotoxins should help in the construction of agonists and antagonists of enterotoxin action, thereby providing a mechanism for positive or negative regulation of enterotoxin action. Finally, the proposed studies are important because they will provide information on the structural basis of action of the most potent T-cell mitogen and lymphokine inducer known.